Lower carboxylic acids and esters such as acetic acid and methyl acetate have been known as industrial chemicals for many years. Acetic acid is used in the manufacture of a variety of intermediary and end-products. For example, an important derivative is vinyl acetate which can be used as monomer or co-monomer for a variety of polymers. Acetic acid itself is used as a solvent in the production of terephthalic acid, which is widely used in the container industry, and particularly in the formation of PET beverage containers. There has been considerable research activity in the use of metal catalysts for the carbonylation of lower alkyl alcohols, such as methanol, and ethers to their corresponding carboxylic acids and esters
Carbonylation of methanol is a well known process for the preparation of carboxylic acids and particularly for producing acetic acid. Such processes are typically carried out in the liquid phase with a catalyst. The prior art teaches the use of a number of catalysts for the synthesis of carboxylic acids by reaction of alcohols with carbon monoxide at elevated temperatures and pressures using a fixed bed reactor in both gas and liquid phase reactions. Generally, the liquid phase carbonylation reaction for the preparation of acetic acid using methanol is performed using homogeneous catalyst systems comprising a Group VIII metal and iodine or an iodine-containing compound such as hydrogen iodide and/or methyl iodide. Rhodium is the most common Group VIII metal catalyst and methyl iodide is the most common promoter. These reactions are conducted in the presence of water to prevent precipitation of the catalyst.
Currently, the best industrial practices for the carbonylation of methanol to acetic acid uses homogeneous catalysts consisting of either a mixture of rhodium and lithium, as exemplified in U.S. Pat. No. 5,510,524, or a mixture of iridium and ruthenium, as exemplified in European Patent Application EP 0 752 406 A1.
Unfortunately, these catalysts suffer from the typical difficulties associated with the use of homogeneous catalysis. In particular, upon separation of the catalyst and liquid components, catalyst precipitation and volatilization can occur, particularly if one tries to remove most of the liquid component. Further, mass transfer limitations, which are inherent in the transfer of gaseous carbon monoxide into a liquid reaction medium, limit the ultimate achievable rates in these homogeneously catalyzed processes.
U.S. Pat. No. 5,144,068 describes the inclusion of lithium in the catalyst system which allows the use of less water in the Rh-I homogeneous process. Iridium also is an active catalyst for methanol carbonylation reactions but normally provides reaction rates lower than those offered by rhodium catalysts when used under otherwise similar conditions.
U.S. Pat. No. 5,510,524 teaches that the addition of rhenium improves the rate and stability of both the Ir-I and Rh-I homogeneous catalyst systems.
European Patent Application EP 0 752 406 A1 teaches that ruthenium, osmium, rhenium, zinc, cadmium, mercury, gallium, indium, or tungsten improve the rate and stability of the liquid phase Ir-I catalyst system. Generally, the homogeneous carbonylation processes presently being used to prepare acetic acid provide relatively high production rates and selectivity. However, heterogeneous catalysts offer the potential advantages of easier product separation, lower cost materials of construction, facile recycle, and even higher rates.
Schultz, in U.S. Pat. No. 3,689,533, discloses using a supported rhodium heterogeneous catalyst for the carbonylation of alcohols to form carboxylic acids in a vapor phase reaction. Schultz further discloses the presence of a halide promoter.
Schultz in U.S. Pat. No. 3,717,670 describes a similar supported rhodium catalyst in combination with promoters selected from Groups IB, IIIB, IVB, VB, VIB, VIII, lanthanide and actinide elements of the Periodic Table.
Uhm, in U.S. Pat. No. 5,488,143, describes the use of alkali, alkaline earth or transition metals as promoters for supported rhodium for the halide-promoted, vapor phase methanol carbonylation reaction. Pimblett, in U.S. Pat. No. 5,258,549, teaches that the combination of rhodium and nickel on a carbon support is more active than either metal by itself.
Of these active carbonylation catalysts, carbon based supports are generally substantially better from a rate perspective, with Ni, Sn, and Pb displaying negligible activity on inorganic oxides. The normally large difference in rates upon changing from and activated carbon to an inorganic support has been exemplified in in M. J. Howard, et. al., Catalysis Today, 18, 325 (1993), where, on p. 343, a mixed Rh--Ni catalyst on activated carbon support can be compared to a rhodium on inorganic oxides. With the Rh--Ni on activated carbon, the rate is reported as being ca. 5 mol of acetyl/g of Rh/h at 188.degree. C., 9 bar of 1:2 CO:H.sub.2, whereas the range for inorganic oxides is only 0.1 to 0.5 mol of acetyl/g of Rh/h despite being operated at substantially higher temperature (220.degree. C.) and substantially higher CO pressures (40 bar CO pressure).
Evans et al., in U.S. Pat. No. 5,185,462, describe heterogeneous catalysts for halide-promoted vapor phase methanol carbonylation based on noble metals attached to nitrogen or phosphorus ligands attached to an oxide support.
Panster et al., in U.S. Pat. No. 4,845,163, describe the use of rhodium-containing organopolysiloxane-ammonium compounds as heterogeneous catalysts for the halide-promoted liquid phase carbonylation of alcohols.
Drago et al., in U.S. Pat. No. 4,417,077, describe the use of anion exchange resins bonded to anionic forms of a single transition metal as catalysts for a number of carbonylation reactions including the halide-promoted carbonylation of methanol. Although supported ligands and anion exchange resins may be of some use for immobilizing metals in liquid phase carbonylation reactions, in general, the use of supported ligands and anion exchange resins offer no advantage in the vapor phase carbonylation of alcohols compared to the use of the carbon as a support for the active metal component.
Nickel on activated carbon has been studied as a heterogeneous catalyst for the halide-promoted vapor phase carbonylation of methanol, and increased rates are observed when hydrogen is added to the feed mixture. Relevant references to the nickel-on-carbon catalyst systems are provided by Fujimoto et al. In Chemistry Letters (1987) 895-898 and in Journal of Catalysis, 133 (1992) 370-382 and in the references contained therein. Liu et al., in Ind. Eng. Chem. Res., 33 (1994) 488-492, report that tin enhances the activity of the nickel-on-carbon catalyst. Mueller et al., in U.S. Pat. No. 4,918,218, disclose the addition of palladium and optionally copper to supported nickel catalysts for the halide-promoted carbonylation of methanol. In general, the rates of reaction provided by nickel-based catalysts are lower than those provided by the analogous rhodium-based catalysts when operated under similar conditions.
A number of solid materials have been reported to catalyze the carbonylation of methanol without the addition of the halide promoter. Gates et al., in Journal of Molecular Catalysis, 3 (1977/78) 1-9, describe a catalyst containing rhodium attached to polymer bound polychlorinated thiophenol for the liquid phase carbonylation of methanol. Current, in European Patent Application EP 0 130 058 A1, describes the use of sulfided nickel containing optional molybdenum as a heterogeneous catalyst for the conversion of ethers, hydrogen and carbon monoxide into homologous esters and alcohols.
Smith et al., in European Patent Application EP 0 596 632 A1, describe the use of mordenite zeolite containing Cu, Ni, Ir, Rh, or Co as catalysts for the halide-free carbonylation of alcohols. Feitler, in U.S. Pat. No. 4,612,387, describes the use of certain zeolites containing no transition metals as catalysts for the halide-free carbonylation of alcohols and other compounds in the vapor phase.
U.S. Pat. No. 5,218,140, describes a vapor phase process for converting alcohols and ethers to carboxylic acids and esters by the carbonylation of alcohols and ethers with carbon monoxide in the presence of a metal ion exchanged heteropoly acid supported on an inert support. The catalyst used in the reaction includes a polyoxometallate anion in which the metal is at least one of a Group V(a) and VI(a) is complexed with at least one Group VIII cation such as Fe, Ru, Os, Co, Rh, Ir, Ni, Pd or Pt as catalysts for the halide-free carbonylation of alcohols and other compounds in the vapor phase.
Although many of the earlier catalysts are operable in the liquid phase, the active metal is generally rapidly removed from the support by dissolution in the harsh environments associated with carbonylation of methanol and it derivatives. As a consequence, the reaction becomes a homogeneously catalyzed process despite the presence of the support. Further, even if the association of the metal with the carbon support had been retained, the mass transfer limitations are often exacerbated by the introduction of a third phase into the reactor. The presence of a heterogeneous catalyst in a liquid medium forces CO to, not only diffuse into the reaction medium, but, once in the liquid reaction medium, CO must subsequently diffuse from the reaction medium into the heterogeneous catalyst. As a consequence, the reactions above are all preferably operated in the vapor phase where mass transfer is rapid and leaching is negligible.
To overcome the leaching problem, ligands have been used to bind the rhodium to the catalyst support. For example, U.S. Pat. No. 5,155,261 discloses using amines incorporated within the catalyst to retain the rhodium component on a solid support. Generally these functional groups are incorporated either as part of a resin or by grafting to an oxide support. It is now generally understood that these functional groups are quarternized in the process, forming ammonium salts, and the rhodium, which is present as Rh(CO).sub.2 I.sub.2.sup.-, is bound by electrostatic attraction.
Unfortunately, although the catalysts containing functional groups have been successful in retarding the leaching of the Rh catalyst into the liquid phase, they still do not overcome the problems associated with diffusion. Further, the functional groups, present as quarternary salts, and the resin backbones are subject to thermal degradation placing strict constraints on the operating temperatures that can be employed with these catalysts. The inability to use higher temperatures with these functionalized catalysts seriously limits the ultimate attainable rates when they are employed in carbonylation processes.
These functionalized catalysts have been primarily designed for liquid phase carbonylation and the operation of the functionalized catalysts in the vapor phase would be expected to be difficult given the poor temperature stability of these functionalized catalysts. The poor temperature stability limits the useful pressures and production rates achievable with these functionalized catalysts. Regardless, one of these functionalized catalysts has been tested in the vapor phase. Unfortunately, the rate was only half that of the corresponding liquid phase process.
The much higher rates associated with metals on activated carbon are commercially attractive for a vapor phase carbonylation process. Unfortunately, carbon has several physical limitations which have inhibited its commercial introduction. Although activated carbon is readily available from a number of commercial sources, its characteristics are highly variable, making the generation of reproducible catalysts difficult. Activated carbon is also brittle and has a poor crush strength. As a consequence, it is subject to rapid physical attrition. These physical limitations have apparently prevented the introduction of a vapor phase carbonylation process using metals supported on activated carbon despite the attractiveness of such a process.
Accordingly, there is a need for a carbonylation catalyst which retains the high activity associated with metal catalysts supported on activated carbon, but has greater structural integrity and uniformity associated with harder supports, such as the inorganic oxides.